Beads having an outer abrasive layer of a metal matrix in which diamond grit is embedded appear to be first described in the beginning of the fifties of the previous century (see e.g. U.S. Pat. No. 2,679,839, filed 1952). Such beads are strung onto a steel cord and are separated by means of springs (see U.S. Pat. No. 2,679,839) or by means of a plastic material (see e.g. FR 1.203.000, filed 1958). The plastic material could also be injected between the beads by the aid of a mould (FR 1.203.000, first addition, filed 1959). The ideas of fixing the wire directly to the steel cord e.g. by means of a pin (GB 759,505, filed 1953), by means of soldering (FR 1,265,542, first filed 1960) or by means of swaging (U.S. Pat. No. 3,598,101, first filed 1968) were also explored.
Initially, these sawing cords where used on stationary saws, where they competed with gang saws (reciprocally driven frames with parallel mounted steel lamella on which diamond containing bits are mounted) and circular disc saws. The sawing cord was made in a closed loop by splicing the steel cord as e.g. described in U.S. Pat. No. 2,773,495, filed 1953). After tensioning the loop over two large wheels driven by a motor, the cord could be used as a saw. Current state of the art machines are available wherein up to 80 loops are driven parallel to one another to separate a block of stone into a series of slabs. In an alternative use, such diamond bead sawing cords started to appear in quarries at the beginning of the seventies of the previous century where they were used for block extraction.
A number of methods have been explored by which the abrasive material—mostly diamond—can be fixed onto the beads. There is the method of affixing the diamonds to a metallic tube by means of electrolytic or electroless deposition of nickel (WO 2002/40207). There is also the method of embedding the abrasive particles into a braze that is directly applied on the metal sleeve as described in U.S. Pat. No. 7,089,925.
The method that has become the most successful is via the route of powder metallurgy (described already in U.S. Pat. No. 2,679,839). To this end an annular abrasive element is made from diamond grit that is thoroughly mixed with metal powder and an optional organic wax for forming a paste. The mixture of metal powder normally contains high melting temperature components such as cobalt, tungsten, iron, nickel sometimes in combination with low melting temperature components such as copper, tin, silver to improve consolidation. Possibly compounds such as tungsten carbide can be added to influence hardness and wear of the bead. The mixture is brought in a mould. This preform is sintered into a high density bead by application of pressure (by ram pressing in the mould or by applying isostatic pressure through immersion in a high pressurised fluid) and temperature. Suitable gasses are applied in order to prevent the powder from oxidising during sintering.
In the sintering process the annular abrasive element is heated to a high temperature followed by a slow cooling i.e. in a quasi thermal equilibrium. Even if not all components of the alloy get the chance to melt, the addition of melting point lowering components will lead to intermetallic phases between the different components through diffusion. The metallographic cross section of such annular abrasive element shows therefore a globular and/or granular structure. In the classical bead sintering process it is generally preferred that grain sizes should be small in order to obtain sufficient matrix material hardness. This grain size can be affected by varying the properties of the starting powders and the conditions for their consolidation. In any case grains remain visible in a properly etched metallographic cross section and no directional growth effects are visible. See e.g. “Powder Metallurgy Diamond Tools” by Janusz Konstanty, Chapter 5, 2005, Elsevier Science Title, ISBN 978-1-85617-440-4.
The selection of metals, amount and type of diamonds, and pressure-temperature trajectory is internal know-how of the producers that greatly influence the quality of the final product.
After sintering the annular abrasive element is fixed to a metallic sleeve slightly longer than the abrasive element by means of a braze. The combination of the metallic sleeve with the abrasive element is called a bead. The need for a metallic tube to fix the abrasive element on can be eliminated by forming the abrasive element sufficiently precise as described in US 2007/0194492 A1.
Sintered beads have become the technological leader because:                they have a sufficiently thick abrasive layer        wherein the diamonds are randomly embedded throughout the layer,        the matrix material wears at the same pace as the diamonds are used up        while the matrix material retains the diamonds well and        because the beads can be reproducibly made with narrow geometrical tolerances.        
Thereafter the beads are threaded onto a steel filament carrier cord (‘steel cord’) and subsequently fixed by springs or a plastic. This ‘threading step’ is tedious and time consuming. As the surface of the beads is smooth due to the pressing in a mould, the beads must be ‘dressed’ before being used. This is usually done by using the sawing cord initially at low cutting speed till the abrasive particles are freed from the surface and cut better. Such ‘dressing step’ is time consuming.
Another method for producing abrasive layers is currently making its inroads into the world of saw blades for cutting stone namely laser cladding. In laser cladding a stream of powder is fed into a high-intensity beam of a laser that is focussed on the surface of the substrate by means of a carrier gas stream. The powder is a mixture of metal powder and abrasive particles (usually diamond). The powder melts and forms a pool of molten metal that solidifies and fixes the abrasive particles.
DE 195 20 149 A1 discloses a process for laser-cladding a substrate whereby by means of a cooled (or heated) mould a near final finish of an abrasive or wear resistant surface can be formed. The cladding is applied onto the mould after which the mould is removed. The application only discusses the deposition on relatively bulky substrates such as a saw blade. The cladding is performed in one single layer.
WO 1999/18260 (EP 1027476) describes a cutting tool built on a steel substrate with an abrasive coating comprising abrasive material embedded in a wetting agent containing metal or metal alloy matrix where between the abrasive coating and the steel substrate a non-ferrous layer is present that is substantially free of wetting agent. Although the abstract mentions a saw cable as an example of a cutting tool, this is not further exemplified in the description.
In laser cladding, the low density of diamond relative to that of a molten metal makes the diamonds float upward in the metal pool which leads to a non-homogenous distribution of the diamonds (see FIG. 16 of “Herstellung diamanthaltiger, endkonturnaher, Metallmatrix-Verbundwerkstoffe durch Laserstrahlbeschichten” by A. Lang and H. W. Bergmann in “Material-Wissenschaften and Werkstofftechnologie”, Vol. 27 pp. 215-226, 1996). One solution is to use finer abrasive particles where the up-floating is slowed by the viscosity of the metal pool. But for many technological applications—notably stone cutting—fine diamond particles are not an option.
Another solution is described in WO 1998/15672 where a particular arrangement of vertical substrate surface movement with a laser cladding tool in horizontal direction leads to an upwards rising of the diamond particles in the direction of the deposited layer. Again the deposition is in one layer.
WO 02/06553 discloses a method for making sawing beads according the laser cladding method. No details are given on the metallographic structure resulting from the method, nor of the materials used. Considerable pre- and post-processing of the resulting carrying tube is necessary in order to obtain the sawing bead.
Although the described methods for making a cutting tool by means of laser cladding sometimes suggest it to be suitable for making beads for sawing cords, this turned out not to be straightforward at all as the inventors experienced. All patents describe the deposition of a laser clad abrasive layer on a massive low carbon steel substrate (such as a sawing disc or tube). In those cases a large heat sink is available to drain off the excess heat.
The inventors faced and solved four major problems                One problem is to drain the heat sufficiently fast from the tiny metal sleeve of less than one gram (!), such that it does not deform or, total disaster, completely melts. On the other hand sufficient heat must be supplied in order to be able to obtain a strong bond to the sleeve and to form a dense abrasive layer. This is called the ‘heating problem’;        The ‘geometry problem’ whereby it turned out to be difficult to produce the beads consistently within geometrical tolerances and with sufficient roundness and centricity. This problem is particularly important as during use the sawing cord must rotate in order to ensure a uniform wear of the abrasive layer.        The ‘particle distribution problem’ whereby it turned out to be difficult to have a uniform distribution of abrasive particles in the abrasive coating. This is important as during use the matrix material gradually abrades away unveiling radial lower laying diamonds in the layer. If all diamonds are e.g. on the surface, they will wear first and no lower laying diamonds are available.        The dressing problem, which is the problem that abrasive particles are buried under a layer of matrix material and are not active from first use onward.        